The invention relates to the purification of hafnium dioxide contaminated with uranium dioxide.
Hafnium dioxide is commercially produced as a byproduct in the manufacture of zirconium from zircon and similar ores. Generally in the course of production, zirconium and hafnium contaminated with impurities such as iron and other metals from the ore and from the processing facility are dissolved in an aqueous acid and then separated in a solvent extraction step as is disclosed by Greenberg U.S. Pat. No. 3,069,232 et al., Miller U.S. Pat. No. 3,006,719 and Overholser U.S. Pat. No. 2,938,769 et al. Typically, the hafnium is recovered from the solvent extraction step as hafnium hydroxide [Hf(OH).sub.4 ] contaminated with from about 0.1 wt % to about 1 wt % uranium dioxide [UO.sub.2 ] based upon the weight of the hafnium hydroxide as hafnium dioxide [HfO.sub.2 ]. The hafnium hydroxide may be calcined and further processed for use in nuclear applications.
The hafnium dioxide from commercial zirconium plants frequently cannot be used in non-nuclear applications because of radiation due to relatively high uranium impurity levels. The uranium impurity level in hafnium dioxide must be reduced to below about 0.005 wt % (as uranium dioxide) before the hafnium dioxide can be released for commercial non-nuclear applications. However, a practical economical hafnium dioxide purification process has not been developed by the art and, heretofore, much of the hafnium dioxide which could not be used in nuclear applications has been buried as low level radioactive waste.